Sound reproducing system



Nov. 5, 1940.

G. L. BEERS ysouND REPRODUCING SYSTEM Filed Dec. 3l, 1956 ."mnnnnnnnll Innnnnnnnr:

(Ittorneg Patented Nov. 5, 1940 Aifui;TENT i oFPicE SOUND REPRoDuciNG'sYsTEM, r'

George L. Beers, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1936, Serial No. 118,436

7 Claims.

This invention relates to signal recording and reproducing sys-tems, and more particularly to a system wherein the linear speed of the record relative to the scanning yelement varies with the playing time.

It is well known that in certain types of phonographic apparatus, as, for example, the conventional phonographs which `employ disc records rotating at constant angular velocity, the linear speed of the record varies relative to the pick-up in accordance with the lradial position of the pick-up on the record. In systems of this type, it has been found that harmonic distortion and loss of high frequencies occur at the low linear speeds obtained on the inside grooves,` and this is particularly true of the so-calledlslowspeed records which are designed to rotate at a speed of about 331/3 R. P. M. Now, this distortion has been found to decrease as the amplitude of the cut of the record grooveis reduced, but, if the entire record is cut at a sufciently low amplitude to minimize the aforementioned distortion, diiiiculty is encountered due to record scratch. Y The primary object of my presentinvention is to provide an improved method of and system for recording signalsupon and reproducing signals from a record element which, during reproduction, varies in linear speed inthe manner previously mentioned, which method and system will berentirely free from the foregoingdiiculties. More specifically, it is an object of my present invention to provid-e an improved method of and system for recording signals upon and/or reproducing signals from a record as aforesaid whereby distortion normally and inherently resulting from the variation in linear speed of the record relative to the reproducing or record scanning element is eliminated.

A further object of my invention is to provide an improved reproducing system as above set forth which, while by no means limited thereto,

is particularly useful in connection with records linear speed relative tov the pick-up is lrelatively high, at normal amplitude, but reduce the amplitude ofthe cutgradually as the pick-up approaches the central portion of the record and the linear speed thereof relative to the pick-up is reduced. Thus, the amplitude of the cut becomes Va function of the linearvelocity of the record groove with respectto the pick-up device, or, otherwise stated, `the attenuation of the excursion of the needle of the pick-up becomes a function 'of the velocity of the record relative thereto and'varies directly with adecrease in velocity. It' follows, therefore, that, as the cente'r of the record ids` approached, the distortion will be materially less than if the groove were cut `with a relatively large constant amplitude, and the reduced output from the record byreason of the reduced amplitude of cut may be compensated for, by` automatically increasing the gain of the amplifier employed in connection with theA pick-up device. The features that I consider characteristic of my invention arest for-th with particularity in the appended claims.` The invention itself, however, both as` toits organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing, in`which A Fig. 1 is a diagrammatic illustrationl of a phonograph employingD a disc record,- and of one amplifier circuit used therewith in accordance with my present invention, Fig. 2`is a similar View showing another form of amplifier which may be employed according to this invention,` r r' l Fig. '3 shows one form of recording system accordingit this invention, and 1 Fig. 4 shows the type of record formed according to this invention. Referring more particularly to the drawing, wherein similar reference characters designate' corresponding parts throughout, I have shown, inyFigure 3, a record blankl which is suitably mounted -for rotation at constant, or uniform, angular velocity andover which a suitable recording element 3 is arranged to move radially bymeans of a feed screw 5 or the like. 'I'he recording element 3 includes a cutter l which is actuated by Vsignal currents derived from a suitable amplier a portion of which is represented by the tube 9. As the record blank l is rotated and the recordingV element moved radially thereover, the cutter formsc Aspiral sound, groove;

therein the amplitude of cut of which is, according to conventional practice, substantially uniform throughout the entire length of the groove. This gives rise to the distortion previously indicated when the record, or a duplicate thereof, is reproduced.

In order to eliminate this source of distortion, at least for practical purposes, the recording element 3 may be provided with a slider II which moves over a suitable resistance element I3 in the plate circuit of the tube 9, the slider II being insulated from the element 3 by an insulating block I2. 'Ihe resistance I3 is of such a value and is so arranged in the plate circuit of the tube 9 that, when the recording is begun adjacent the periphery of the blank I and the slider is at the plate end of the resistor I3, substantially the full signal voltage will be impressed across vthe driving coil (not shown) which actuates the cutter 1. As the recording element 3 moves across the record blank I, however, the slider II -will be moved gradually toward the left hand end of the resistance I3, and this Will correspondingly gradually reduce the voltage across the aforementioned driving coil, whereby the amplitude of eX- cursion of thecutter 1 will be gradually diminished as it approaches the central portion of the record. Accordingly, a record such as that shown in Figure 4 results, this record being shown illustratively with four turns A, B, C and D of the recorded groove chosen at random. It will be noted that the groove portion A, at which the linear velocity of the record I relative to the cutter 1 is greatest, has the greatest amplitude of cut; the groove portions B and C, Where the record has relatively lower linear speeds with respect to the cutter 1, have progressively smaller amplitudes of cut; and finally, at the groove portion D, where the linear lspeed of the record I with respect to the cutter 1 is the least, the amplitude of cut is the smallest of the four groove turns shown. K

The recording may, of course, be commenced adjacent the central region of the record and the cutter moved radially outwardly thereover instead of inwardly, as described. In such case, the slider II will move from adjacent thealeft hand end of the resistance I3-to the right hand end thereof, and thevoltage impressed across the driving coil for the cutter 1 will be gradually increased. Thus, starting with a low amplitude of cut in the central region of the record blank I, the amplitude of cut will gradually increase as the periphery of the blank I is approached, and the resulting record will again be like that shown in Figure 4.

For reproducing a record so formed, a system such as that illustrated in Figure 1 may be employed. The record 2I which, in actual practice, is a duplicate of the record I, is rotated by suitable means at constant angular velocity (for example, 331/3 R. P. M Pivotally mounted at 23 is an arm 25 which carries a suitable record scanning element 21, such as an electro-magnetic pick-up device provided with a needle 29 for tracking the record groove as the device moves substantially radially over the record about the pivot 23. The output of the pick-up 21 is fed, through an input transformer 3|, toa suitable amplifier 33 shown as a two-stage amplier merely for the sake of illustration and including an amplifier tube 35 in the first-stage and an amplifier tube 31 in the second stage, an output transformer 39 coupling the amplifier 33 t0 a loudspeaker not shown) Connected to the pick-up arm 25 but insulated therefrom by an insulating block 4I is a contact arm or slider 43 of a potentiometer having a resistance element 45 over which the arm 43 slides. The resistance element 45 is connected in the plate circuit of the tube 35 and is so arranged that the audio frequency potentials from the pick-up device 21, after being amplified by the tube 35, are impressed thereacross. As the pickup 21 moves toward the center of the record 2I, the slider 43 is moved toward the right hand, or plate, end of the resistor 45, and the amplication of the tube 35 is caused to increase. Hence, while the power which the pick-up device 21 receives from the record gradually diminishes as it approaches the central region thereof by reason of the gradually .reducing amplitude of cut of the sound groove, and therefore less power is gradually fed to the amplifier 33, the gain of the amplifier 33 is simultaneously correspondingly increased as the arm 43 slides toward the plate end of the resistor 45, whereby the output of the amplier may be maintained substantially uniform.

The value of the resistor 45 and a condenser 41 in the coupling circuitbetween the amplier tubes 35 and 31 may be so proportioned that the high frequencies will be attenuated as the amplier gain is increased. This will prevent the reproduction of objectionable scratch. The loss in iidelity which may result from the attenuation of the 'high frequenciecs will probably not be noticed by the average listener, whereas the distortion resulting, from a high amplitude cut in the region of the groove turns C and D might be quite objectionable. The advantages of my invention, from this point of view, will, therefore, at once be apparent. n

In Figure 2, I have shown another form of amplier system which may be used advantageously according to my invention. This system employs an exponential tube 5I, such as a Type 35 tube, for example, in the grid circuit of which the potentiometer 43--45 is connected through the secondary winding of the transformer 3l. A C-battery 53A is connected across the resistance element 45 and applies a bias to the grid Sla. of the tube 5I. As the pick-up arm 25 moves inwardly over the record, the slider moves toward the lament end of the resistance 45 and varies the bias on the grid 5Ia to vary the amplification of the tubev 5I inversely as the amplitude of the cut of the recorded groove.

It will be apparent to those skilled in the art that many further modifications oi my invention are possible, both in recording systems and in reproducing systems, as well as in the record itself. For example, the invention may, obviously, be expeditiously applied to photographic records either upon discs designed to rotate in a manner similar to the discs I and 2|, or upon lm strips which are designed to move at a variable linear speed past a stationary scanning member, in whichcase the iilm itself may be relied upon to vary the gain of the amplier. It should also be apparent-that, While I` have described the record 2 I as being one from which reproduction commences at a point adjacent the periphery thereof and terminates at a point in the region of the center thereof, so that the pickup 21 moves substantially radially inwardly thereover, the record may equally well be one from which the reproduction is begun from the central region and the pick-up 21 is caused to move radially outwardly thereover. In such case,

the slider 43 will, of course, move gradually from the right hand end of the resistance element 45 toward the left hand end thereof, with a corresponding reduction in gain of the ampliiier as the needle 29 is made to continuously increase its excursions. In any event, it is essential merely that the gain of the amplier be made to change inversely as the amplitude of cut of the groove being tracked or the photographic record being scanned. Many other changes and modifications will, no doubt, readily suggest themselves to those skilled in the art, and I therefore desire that my invention shall not be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. The method of recording signals upon a rotating record blank over which a cutter is arranged to move in a, radial direction which comprises commencing the recording adjacent the periphery of said blank with a predetermined amplitude of excursion of the cutter, causing said cutter to move radially inwardly across said blank and in contact therewith while simultaneously causing said blank to rotate at a constant angular velocity, and gradually diminishing the amplitude of excursion of said cutter in accordance with the instantaneous linear velocity of said blank relative to said cutter as said cutter approaches the center of the blank.

2. The method of recording signals upon a rotating record blank over which a cutter is arranged to move in a radial direction which comprises commencing the recording adjacent the center of said blank with a predetermined amplitude of excursion of the cutter, causing said cutter to move radially outwardly across said blank and in contact therewith while simultaneously causing said blank to rotate at a constant angular velocity, and gradually increasing the amplitude of excursion of said cutter in accordance with the instantaneous linear velocity of said blank relative to said cutter as said cutter approaches the periphery of the blank.

3. A sound record of the type adapted to be reproduced by a scanning element with respect to which the record moves at a varying linear speed during reproduction characterized in that thev amplitude of recorded signal thereon varies progressively from one point therein to another independently of the frequency of the recorded signal, the amplitudes being greatest at the point where the record moves fastest relative to said scanning element and being least at the point where the record moves slowest relative to said scanning element.

4. A second record according to claim 3 characterized in that the amplitude of the recorded signal is, at any point therein, a function of the instantaneous linear velocity of said record relative to said scanning element at said point.

5. In a. method of reproducing sound from a record having a groove which varies progressively in amplitude of recorded signal along its length, the step which comprises progressively varying the amplitude of reproduction in accordance with the amplitude of said groove.

6. In the method of reproducing sound from a record having a groove which varies progressively in amplitude of recorded signal along its length, the step which comprises progressively varying the amplitude of reproduction inversely as the amplitude of said groove.

7.`The method of electrically reproducing sound from a record wherein the signals are recorded with progressively changing amplitudes along the length thereof which comprises bringing a scanning element into scanning relation with said record, causing said record to'move relative to said element at a linear speed which varies as a function of the amplitudes of the recorded signals, feeding the output from said scanning element to an electrical amplifier, and causing the gain of said amplier to vary in accordance with the amplitude of the instantaneous record portion being reproduced.

GEORGE L. BEERS. 

